This invention relates to a vehicle radar apparatus which transmits and receives radar waves and is capable of detecting an objective existing around a vehicle based on reflected radar waves.
Conventionally, a FMCW type radar apparatus is known as a representative vehicle radar apparatus which transmits, as a signal carried on the radar wave, a triangular wave transmission signal fs having the frequency gradually increasing and decreasing cyclically as shown in FIG. 7A and receives radar waves reflected from an objective to generate a received signal fr (refer to the Japanese Patent Application Laid-open No. 2001-166042). This radar apparatus obtains frequencies fbu and fbd shown in FIG. 7B based on mixing of the received signal fr and the transmission signal fs. The frequencies fbu and fbd correspond to the frequency difference between the transmission signal fs and the received signal fr. More specifically, the frequency analysis using a signal processing device or the like is applied to a frequency difference signal (i.e., a beat signal) representing the frequency difference between the transmission signal fs and the received signal fr. As a result of this frequency analysis, the ascending section peak frequency fbu is extracted from an ascending section in which the frequency of transmission signal fs gradually increases, and the descending section peak frequency fbd is extracted from a descending section in which the frequency of transmission signal fs gradually decreases.
As shown in FIG. 7A, when a vehicle installing this radar apparatus and an objective reflecting the radar wave are mutually equal in the shifting speed (i.e., relative speed V=0), the radar wave reflected by the objective arrives the radar apparatus with a delay time equivalent to a time required for going and returning the distance D between the vehicle and the objective. In this case, the received signal fr is substantially coincident with the transmission signal fs if it shifts along the time axis by the amount of this delay time. The ascending section peak frequency fbu is equal to the descending section peak frequency fbd (i.e., fbu=fbd).
On the other hand, when the vehicle installing this radar apparatus and the objective reflecting the radar wave are mutually different in the shifting speed (i.e., relative speed V≠0), the radar wave reflected by the objective is subjected to the Doppler shift in accordance with the relative speed V of the objective. Accordingly, the received signal fr shifts along the frequency axis by the amount of the Doppler shift corresponding to the relative speed V in addition to the amount of delay time corresponding to the distance D of the objective. In this case, the ascending section peak frequency fbu is not equal to the descending section peak frequency fbd (i.e., fb1≠fb2).
In this manner, the received signal fr shifts in the time axis direction as well as in the frequency axis direction in accordance with the distance D and the relative speed V of the objective. In other words, the frequency difference between the transmission signal fs and the received signal fr appearing in the time axis is dependent on the distance D of the objective, while the frequency difference appearing in the frequency axis is dependent on the relative speed V. The frequency fb corresponding to the distance D and the frequency fd corresponding to the relative speed V can be obtained from the following equations 1 and 2.fb=(|fbu|+|fbd|)/2  (1)fd=(|fbu|−|fbd|)/2  (2)
In other words, the frequency fb corresponding to the distance D and the frequency fd corresponding to the relative speed V can be obtained based on the ascending section peak frequency fbu and the descending section peak frequency fbd. Then, from the frequencies fb and fd corresponding to the distance D and the relative speed V, the following equations 3 and 4 can be introduced to calculate the distance D and the relative speed V of the objective.D={C/(4×ΔF×fm)}×fb  (3)V={C/(2×f0)}×fd  (4)where ΔF represents a frequency modulation width of the transmission signal fs, f0 represents a central frequency of the transmission signal fs, fm represents a repetition frequency, and C represents the velocity of light.
In general, each radar apparatus has a limited detection range in which an objective is detectable. The detection range is dependent on the irradiation direction of the radar wave emitted from a transmitting antenna, i.e., dependent on the directivity of the transmitting antenna. Accordingly, the directivity of the transmitting antenna is determined in such a manner that the radar wave has a desired width in each of the vertical direction and the horizontal direction. For example, considering the fact that the vehicle body causes pitching in the up-and-down direction and the road is not always parallel to the radar wave, the directivity in the vertical direction is determined so as to have a certain degree of margin in width
However, as shown in FIG. 8, although the amount is small, the radar wave is inevitably irradiated toward the direction being not aimed. In other words, some of the irradiated radar wave excurses outside the determined irradiation range. If such radar wave is reflected by a road surface near a vehicle body (hereinafter, referred to as “close range road surface”), the reflected wave may be received by a receiving antenna of the radar apparatus. Namely, the receiving antenna possibly receives reflected waves returning from the close range road surface in addition to the reflected waves returning from a preceding vehicle or any other objective ahead of this radar apparatus equipped vehicle.
The received signal, even when carried on the reflected wave returning from the close range road surface, shifts in accordance with the distance and the relative speed between the vehicle and the road surface, and accordingly the frequency difference between the transmission signal and the received signal changes. As shown in FIGS. 9A and 9B, when the system own vehicle is in a stopped condition with a relative speed 0 against the road surface, the ascending section peak frequency fbu1 and the descending section peak frequency fbd1 become the same low frequency (fbu1=fbd1) because the beat signal produced from the transmission signal fs and the received signal fr1 is based on the reflection from the close range road surface.
On the other hand, when the vehicle is traveling and the reflected wave returning from the close range road surface is subjected to the Doppler shift, the received signal fr2 shifts to a position exceeding the ascending section of the transmission signal fs as shown in FIG. 9A. As a result, as shown in FIG. 9C, the ascending section peak frequency fbu2 becomes a negative frequency while both the ascending section peak frequency fbu2 and the descending section peak frequency fbd2 become large in their absolute values.
In this case, it is always recognized in the radar apparatus that some of the radar waves emitted from the transmitting antenna directly sneaks into the receiving antenna. Thus, as shown in FIG. 10A, the receiving intensity (i.e., receiving power) is maximized at an extremely low-frequency region. The received signal, carried on the reflected wave returning from the close range road surface under the condition that the vehicle is stopped, is completely involved or concealed in this extremely low-frequency region. Thus, the receiving intensity of the received signal from the close range road surface is not detectable through the frequency analysis or comparable signal processing.
However, when the vehicle is traveling, the received signal fr2 carried on the reflected wave returning from the close range road surface is subjected to the Doppler shift as described above. Accordingly, both of the ascending section peak frequency fbu2 and the descending section peak frequency fbd2 of the beat signal shift out of the extremely low-frequency region as shown in FIG. 10B. In this case, the ascending section peak frequency fbu2 becomes a negative frequency. However, the radar apparatus calculates it as a positive frequency because the negative frequency is processed as having an inverse sign in the frequency analysis or other signal processing.
It is needless to say that the radar apparatus should not identify a close range road surface with the objective to be detected. Accordingly, executing the processing based on the peak frequencies fbu2 and fbd2 originated from a close range road surface results in erroneous detection of the objective.
Similarly, when it is raining, there is the possibility that radar apparatus receives the reflected wave returning from raindrops. In other words, the reflected wave returning from the raindrops will cause erroneous detection of the objective. Especially, the received signal carried on the reflected wave returning from the raindrops tends to have a higher receiving intensity compared with the received signal carried on the reflected wave returning from a close range road surface. Thus, the possibility of erroneously detecting the objective will increase.
FIGS. 11A, 11B, and 11C show power spectrums representing the intensity of each frequency component of the beat signal in the frequency ascending section as well as in the frequency descending section. FIG. 11A shows a power spectrum in a vehicle stopped condition, FIG. 11B shows a power spectrum in a vehicle traveling condition on a dried road surface, and FIG. 11C shows a power spectrum in a rainy vehicle traveling condition. As understood from FIGS. 11A and 11B, a peak frequency not being recognized in the vehicle stopped condition appears in the low-frequency region once the vehicle starts traveling. Furthermore, as shown in FIG. 11C, the peak frequency appearing in the low-frequency region has large peak intensity when it is raining.
To solve this problem, using a high pass filter to cut the peak frequency in the low-frequency region is conventionally proposed. However, this is not preferable because an objective residing in the vicinity of the vehicle becomes undetectable.